The field of the present invention relates to an X-ray CT (Computed Tomography) apparatus, a method of controlling the same, and a program. More particularly, the invention relates to an X-ray CT apparatus for sequentially performing a first scan and a second scan using different tube voltages or different tube currents by switching tube voltage or tube current of an X-ray tube, on the same slice in a subject, a method of controlling the same, and a program for the same.
Hitherto, as a scan method of an X-ray CT apparatus, for example, there is a known method of performing a scan by applying two kinds of X-rays using different tube voltages of an X-ray tube, that is, X-ray energy distributions different from each other to a subject. By the scan, two kinds of slice images of the same slice in a subject in which contrast appears different according to the kind of a tissue as a component of the subject are acquired. A subtracting process is executed on the two kinds of images to obtain a differential image in which a specific tissue is emphasized or from which a specific tissue is eliminated.
An example of the scan method for acquiring two kinds of images is a method disclosed in Japanese Unexamined Patent Publication No. 2006-187453. A plurality of data collecting systems each made of an X-ray tube and an X-ray detector are mounted on an X-ray CT apparatus. Tube voltages different from each other are set in the X-ray tubes, and the same slice in the subject is scanned simultaneously by the data collecting systems (first scan method). According to another method described in Japanese Unexamined Patent Publication No. 2006-006531, first and second scans using different tube voltages are performed by switching the tube voltage of the X-ray tube on the same slice in the subject (second scan method).
In the case of comparing the two kinds of images, desirably, a change in the flow of a contrast agent in the subject and body motions caused by heart beat, breathing, and the like are small in two slice images obtained by scanning an almost the same position in a subject with two kinds of different tube voltages.
In the first scan method, the subject is scanned simultaneously with two kinds of X-rays, so that there is no change in the flow of the contrast agent in the subject and no body motion caused by heart beat, breathing, and the like in the acquired two kinds of images. Therefore, from the above-described viewpoint, it can be said that the first scan method is ideal. On the other hand, there are drawbacks such that the first scan method cannot be applied to an existing X-ray CT apparatus having only one data collecting system made of the X-ray tube and the X-ray detector or, when two or more data collecting systems each made of an X-ray tube and an X-ray detector are mounted, the cost largely increases.
On the other hand, the second scan method has advantages such that the method can be applied to an existing X-ray CT apparatus having only one data collecting system made of an X-ray tube and an X-ray detector and modification in hardware is hardly required, so that cost does not increase. Recently, time required for a scan is getting shorter. The shortest time is about 0.35 second. It is therefore becoming almost unnecessary to consider the influence of performing a scan two times at different timings on the demand for reducing a change in flow of the contrast agent in the subject and the body motion.